I have a Downtube Mini folder with 16" wheels. I added a rack from a Dahon Curve - it's sized to fit over the 16" wheels so it is useless for mounting panniers. Too low to the ground, too many problems with heel strikes.

I needed to carry my laptop and clothes to work, so I added a Wald 585 Rack Top Basket. It's ziptied on there to within an inch of its life. Very secure. I like the Wald basket - not how it looks, but the functional storage it provides. It doesn't interfere with the bike fold, and I know my stuff is going to stay in there.

But now I'm getting greedy. I want more storage. A right-side pannier would be perfect, it still wouldn't interfere with the fold, and if the pannier is mounted high enough and far back enough, it wouldn't cause trouble.

The Wald basket has a top rim, let's say 4mm diameter metal tubing around the top. Not as thick as rack tubing.

Could I attach a pannier to the right-side-top of the Wald basket? Which attachment system, if any, would work? Anyone have any experience with this?

I would not imagine you would want to do that with the basket the way it is attached. If you can permanently attach the basket to the rack with hardware instead of bungees MAYBE you could attach a pannier. However, with some weight in the pannier you center of gravity would be quite high and the handling could suffer.